1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method that perform averaging processing, screen processing, and resolution converting processing to a target image, which is a processing target.
2. Description of Related Art
If screen processing is performed to an original image in order to express a half tone when the original image is printed (see, for example, Japanese Patent Application Laid-Open Publication No. 2001-309183), a peculiar repetitive pattern called as moiré is sometimes generated in an output image by interference between the period of the halftone dot structure of the original and the period of a pattern applied in the screen processing. In order to prevent the generation of the moiré, it is effective to put the period structure on the original side into disorder, and averaging processing has hitherto been performed for the purpose of putting the period structure into disorder.
However, because the averaging processing averages the detail parts (particulars) of an image together with the period structure of the original image, the averaging processing has a problem of reducing the sharpness of the image. Accordingly, the methods of according the period of the averaging processing with that of the screen processing for holding the sharpness were disclosed (for example, see Japanese Patent Application Laid-Open Publications Nos. 2004-364084 and 2006-14262).
However, it has become possible to heighten the resolution of an output image by the advancement of the printing technique, and the present state of things is that the resolution at the time of outputting is higher than that at the time of reading. An image processing apparatus performs resolution converting processing to an original image after reading to produce an original image having higher resolution and further performs screen processing to the processed original image to produce an output image. At this time, the order of the processing becomes a problem. That is, because the image data quantity to be processed at a subsequent stage increases by the heightened resolution if resolution converting processing is performed at first, a line buffer memory for holding data should have a large capacity. For example, if resolution is converted from 600 dpi to 1,200 dpi, averaging processing is performed using a matching table for 6′6 pixels as shown in FIG. 19A before resolution conversion, and consequently line buffer memories for five lines are sufficient. However, because the size of the matching table becomes 12′12 pixels as shown in FIG. 19B to be twice after resolution conversion, line buffer memories for 11 lines become necessary. This increases the cost and delays processing.
The Japanese Patent Application Laid-Open Publications Nos. 2001-309183, 2004-364084, and 2006-14262 do not disclose any configuration considering the problems owing to the resolution conversion. The image processing apparatus described in the Japanese Patent Application Laid-Open Publication No. 2001-309183 is provided with a plurality of image memories arranged between two pieces of filter processing, but such a configuration expands the circuit size and cannot resolve the heightening of cost accompanying the expansion of the circuit size.